Amplifiers having gains which are controlled by the magnitude of a control signal are utilized in a variety of applications. More specifically, quadraphonic-stereo systems, for instance, sometimes include voltage controlled amplifiers (V.C.A.) which must provide an output signal having an amplitude which is a function of a control signal. Also, such amplifiers can be used in automatic gain control (A.G.C.) circuits which provide an output signal having a constant amplitude even though the input signal magnitude varies. A.G.C. circuits have decreased gain in response to a control signal magnitude indicating that the amplitude of the output signal is tending to increase and an increased gain in response to a control signal magnitude indicating that the amplitude of the output signal is tending to decrease.
Some prior art A.G.C. and V.C.A. circuit configurations have signal transfer characteristics which are linear enough for amplifying signals having small amplitudes on the order of millivolts but which are too non-linear for amplifying signals having large amplitudes on the order of volts as used in high quality sound reproducing equipment. Moreover, other prior art A.G.C. and V.C.A. circuits, while providing sufficiently linear signal transfer characteristics, allow the quiescent or direct current (d.c.) component of the output voltage at an output terminal thereof to change with variation in the magnitude of the gain control signal. These output voltage variations result in unwanted sounds being produced by loud speaker systems. Moreover, the resulting shift in the output voltage level complicates the design of subsequent stages being driven by the gain controlled amplifier. Also the shift in output voltage level restricts the maximum excursion that the output voltage can have without being distorted.
In response to the present emphasis on miniaturization and because of many known advantages of integrated circuits, such as reliability, cost reduction and compactness, it is desired to provide gain controlled circuits in monolithic form. Prior art gain controlled circuits requiring coupling capacitors do not lend themselves to direct coupling to other stages which is desired in integrated circuit design. Furthermore, prior art integrated circuits providing gain control also sometimes include differential current sources requiring a plurality of active components. It is desirable to reduce the number of active components in integrated circuit structures to improve yield and thus reduce the cost so long as the elimination does not adversely affect circuit performance. Also some prior art integrated gain controlled amplifiers are not suitable for being driven by a single-ended input signal thereby decreasing their usefulness in many applications.